1. Field of the Invention
This invention relates to weld inspection systems and more specifically to an efficient, non-contacting, laser-ultrasonic weld inspection system for on-line and off-line use.
2. Description Of the Related Art
Automated welding systems are commonly used in many industrial applications. During the welding process, various factors can introduce structural flaws and defects in the weld, causing a reduction in the fusion width, and hence the strength and reliability of the welded part.
These defects are often hidden and impossible to observe visually. For example, the fusion width of the weld band may be of an incorrect size, the defect could be a small crack in the weld, or the weld itself may be located between layers of opaque material. In addition, typical automated weld systems operate at high throughput speeds. Therefore, a weld defect caused by a malfunction in the welding system could be reproduced many times before the problem is discovered.
Consequently, there is a need for reliable and accurate weld inspections systems that are fast enough to keep up with automated welding systems. Ideally, the weld inspection system should be capable of on-line operation to diagnose faulty welds early in the welding process. In addition, the on-line system should be capable of use as a transducer in a closed-loop welder control system, so that each weld is optimally formed and is certified to a given specification. The system should also have high spatial resolution in order to detect very small defects in the weld that would otherwise go undetected.
There are several off-line and on-line techniques available for weld inspection. One off-line technique involves taking sample parts from the assembly process on a periodic basis and analyzing the welds. Frequently, the analysis involves breaking or sawing through the weld itself to determine its strength and/or size. However, this technique destroys potentially useful parts in the process. In addition, since only the samples are analyzed, many untested welds or weld areas (which may exist immediately adjacent to the sectioned region) are allowed to pass through untested, resulting in a statistically unreliable evaluation procedure.
Some of the prior on-line techniques, such as that disclosed in U.S. Pat. No. 5,121,339, entitled "LASER WELD FAULT DETECTION SYSTEM", issued Jun. 9, 1992 to D. Jenuwine, et. al., analyze the weld during the welding process by sampling and analyzing high frequency emissions that emanate from the weld itself. The high frequency emissions are sampled either directly by using ultrasonic sensors that are placed in physical contact with the part being welded, or indirectly by non-contacting sensors that sense and analyze the airborne emissions from the weld.
The contacting acoustic emission detection systems are expensive, unreliable, and difficult to set up and calibrate. The non-contacting emission detection systems are difficult to calibrate, especially if weld conditions are not identical from part to part. In addition, the non-contacting systems are very sensitive to ambient noise, which can reduce the accuracy of the weld analysis.
Neither of these emission detection systems are very effective in giving detailed information about the weld. They are more useful for qualitative pass/fail determinations rather than quantitative, high resolution information about the weld.
Furthermore, existing contacting and close proximity systems can not be readily implemented in the adverse environments that are typically found in the case of in-factory operation. These environments may include high temperature, plasma, vacuum, or radiation environments. In addition, existing systems are not robust, because they must be "matched" to the surface contours of a given workpiece.
Laser-ultrasonic techniques have been used in both on-line and off-line systems. For some examples of laser-ultrasonic flaw detection techniques, see C. B. Scruby and L. E. Drain, Laser Ultrasonics, Techniques and Applications, Adam Hilger, New York (1990), pages 325-350. The technique typically used by prior laser-ultrasonic systems is illustrated in FIGS. 1aand 1b.
A pulsed transmitter laser beam 20 is focused to a single spot 22 on the outer surface 24 of a workpiece 26, typically made of metal, with an internal crack or flaw 32. The transmitter beam 20 generates acoustic waves 28 in the first workpiece 26 which propagate over a broad angular range, so that a portion of the waves 28 illuminate the flaw 32. The acoustic waves 28 are reflected by the flaw 32 to a second workpiece surface 30, and cause the surface 30 to vibrate. A read-out laser beam 36 is focused to a spot 38 on the second workpiece surface 30. The vibrations induced by the reflected acoustic waves 31 phase modulate the read-out beam 36, which is reflected by the second surface 30. The reflected, modulated read-out beam is then optically interfered with a reference beam (not shown) or is directed to a frequency discriminator, such as a Fabry Perot cavity, and the resulting interference pattern is analyzed by receiver electronics to extract information about the existence and size of the flaw 32.
Since the amount of acoustic energy that is reflected to the second workpiece surface 30 increases as the size of the flaw 32 increases, this technique can be used to determine both the existence and the size of the flaw 32, as illustrated by the example of a signal vs. time plot of FIG. 1b.
One of the problems with this technique is illustrated in FIGS. 1c and 1d. In many cases, parasitic acoustic coupling paths are present in the system. These parasitic paths could be formed by reflection of acoustic waves from an edge 39 of the workpiece 26, or from other flaws. The parasitic acoustic waves 42 that are reflected from the edge 39 can interfere with or even dominate the acoustic signal detected by the read-out beam 36. This could greatly disturb and, in some instances, completely void the measurement process as illustrated in the signal vs. time plot of FIG. 1d.
Another problem with prior laser-ultrasonic systems that utilize interferometric laser receivers is that the efficiency of the coherent detection required for interferometric measurements is greatly reduced when the read-out beam is reflected from the rough surface of the welded workpiece. Since the reflected read-out beam is being coherently combined with a reference beam, the read-out beam must have temporal and spatial coherence relative to the reference beam. The reflection from the rough surface produces a "speckle" field distribution on the optical detector that is used to detect the interference pattern. The spatial coherence of the reflected read-out beam is only maintained over a single "speckle" width. If the phase aberrations on the reflected read-out beam are not corrected (or compensated), only a small part or, equivalently, a single spatial mode of the reflected read-out beam will be able to coherently combine with the reference beam (only the area of the reflected read-out beam over which spatial coherence is maintained). The resulting detector signal is thousands of times weaker than it would have been if the surface of the welded workpiece had been a perfect mirror surface.
There are several schemes which employ some form of robust laser ultrasonic receiver to sense minute vibrations at high bandwidths in the presence of rough-cut workpiece surfaces:
(1) Phase-Conjugate Compensation Scheme
Systems that utilize phase-conjugate compensation schemes, such as those described in Paul et al., "Interferometric detection of ultrasound at rough surfaces using optical phase conjugation", Applied Physics Letters, Vol. 50, pages 1569-1571 (1987), and Matsuda et al., "Optical Detection of Transient Lamb Waves on Rough Surfaces by a Phase-Conjugate Method", Japanese Journal of Applied Physics, Vol. 31, pages 987-989 (1992), utilize a double-pass optical architecture in which a laser probe beam illuminates the workpiece surface under inspection. The probe beam is modified temporally (by the desired ultrasound) and spatially (by the rough workpiece surface). The probe beam portion that is scattered and/or reflected by the workpiece surface is directed onto a phase-conjugate mirror. The conjugate wave (wavefront-reversed replica of the scattered and/or reflected probe beam) then retraces its path back to the workpiece surface and, after reflection from the surface, has its spatial wavefront restored back to its initial (planar) wavefront. However, the conjugate wave (return beam) is now "doubly" encoded with the desired ultrasound information as a result of the two reflections from the workpiece surface. The fact that the return beam is now planar enables one to more efficiently detect the ultrasound via coherent detection techniques.
There are drawbacks to this approach. First, unless all the scattered light is collected, the return beam's wavefront will not be perfectly restored to its original planar shape. Second, the beam must be reflected twice off the rough workpiece surface. If the workpiece surface has low reflectivity, the detected optical power will be greatly reduced. If there are local reflectivity "drop-outs" on the workpiece surface (due to scratches, digs, rust spots, blemishes, etc.), the spatial amplitude drop-outs will be impressed onto the double-reflected return beam, resulting in a reduction in the sensitivity of the system.
(2) Two-Wave Mixing Schemes
In systems that utilize a two-wave mixing scheme, such as the ones described in Ing et al., "Broadband optical detection of ultrasound by two-wave mixing in a photorefractive crystal", Applied Physics Letters, Vol. 59, pages 3233-3235 (1991), and Blouin et al., "Detection of ultrasonic motion of a scattering surface by two-wave mixing in a photorefractive GaAs crystal", Applied Physics Letters, Vol. 65, pages 932-934 (1994), only a single pass off the workpiece surface is required. The reflected signal beam is combined in a photorefractive crystal with a planar "pump" beam. Energy from the planar pump beam is diverted in the direction of the aberrated signal beam. The pump beam forms the local oscillator, so that coherent detection (either homodyne or heterodyne) can be performed. These schemes suffer from several problems. First, the pump beam must be coherent with respect to the signal beam over a period of time equal to the response time of the photorefractive crystal. Coherence is required to form the photorefractive gratings required for energy exchange between the beams.
Second, if the reflected signal beam going into the photorefractive crystal is highly speckled, the resultant output beam will likewise be speckled, causing amplitude fluctuations that reduce the signal-to-noise ratio by a factor of approximately 2. In addition, the intensity fluctuations that also arise from speckle may cause local depletion effects that adversely affect the wavefront matching between the signal and diverted pump waves. This will create a spatial mismatch between the signal and pump waves, which will result in reduced coherent detection sensitivity.
(3) Fabry Perot Cavity Scheme
Systems that utilize this scheme, such as the one described in Monchalin, "Optical detection of ultrasound at a distance using a confocal Fabry-Perot interferometer" Applied Physics Letters, Vol. 47, pages 14-16 (1985), utilize a Fabry Perot interferometer, which is basically a time-delayed, self-referencing interferometer (also called a "discriminator"), whose output is proportional to the velocity of the surface under inspection. In contrast, the two schemes described above measure the displacement of the workpiece surface. The Fabry Perot scheme has many drawbacks. First, the output response is only linearly proportional to the workpiece surface velocity over a finite bandwidth. Second, the field-of-view is relatively small. Third, the Fabry Perot cavity length must be long enough so that its free spectral range is compatible with the bandwidth of the signal to be detected. This results in relatively long devices (typically longer than one foot). Fourth, servo controls are needed to properly stabilize the Fabry Perot cavity length to the correct operating (bias) point.